Why nested scope rules do not apply to inner Class?

[Maric Michaud]

Le Tuesday 12 August 2008 23:15:23 Calvin Spealman, vous avez écrit :
> The issue has been brought up several times before. There have been
> proposals to make nested classes better supported, but they are always
> shut down. The vote is always against it. Your metaclass example is
> one thing, but its not really applicable here. I've never seen or
> would encourage nested classes even in that case. The metaclass is
> typically defined on its own, and then referenced by one or more
> classes that use it. Defining it as a nested class saves you one line
> of code, but IMHO makes the result just a bit more cluttered, while
> reducing the elegance of reusing the metaclass.
>
> Here are only a few examples of threads giving good reasons against
> class nesting. I've never seen any good arguments for it. There are
> dozens of good reasons we don't encourage it and won't actively
> support it.
>
> http://mail.python.org/pipermail/python-dev/2005-March/052454.html
> http://mail.python.org/pipermail/python-dev/2002-November/029872.html
>

I was not aware of any "nested classes are unsupported" before and didn't 
consider nested classes as bad practice till now, even with the pickle 
limitation (not every class are intended to be pickled), more you didn't give 
any evidence or any pertinent quote of this and at least one of Guido in the 
above threads seems contradict you :
http://mail.python.org/pipermail/python-dev/2002-November/029906.html

BTW my concern was not really about nested classes but the asymetry between 
function scope and classes scope, there are at least two examples that come 
to my mind and for which I don't see good argument to keep the situation as 
it is. First is the very odd behavior of genexp, second is the dynamic type 
factory, I know it's not a big deal to in each case and can live with this, 
but these pieces of code fail unexpectedly while even a experimented python 
programmer would have thought they work at first glance, and the explanation 
of why they fail to a newcomer is not easy (specially for the first)

(1) this fail  only on the last line while it would have work well with 
listcomp and works well in a function definition
class A(object) :
    l = range(5)
    m = list(a+b for a in l for b in range(5))
    n = list(a+b for a in range(5) for b in l)

(2) This won't work as it would with nested functions, you need to build the 
new calss directly with type('dynamic', (object,), {"type_": type_})

def create_type(type_) :
    class dynamic(object) :
        type_ = type_
    return dynamic


-- 
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Maric Michaud